ETBE is a component for increasing the octane number in fuels for gasoline engines. ETBE is increasingly being used as an octane number improver in place of MTBE, because there is discussion about the use of MTBE in fuels due to possible groundwater contamination. In addition, the use of ETBE is required in some countries, for example Germany, when bioalcohol, i.e. ethanol, prepared from renewable raw materials (plants) is used to prepare ETBE.
ETBE can be obtained from C4 olefin mixtures, for example from C4 cuts from steamcrackers. These mixtures consist substantially of butadiene, the monoolefins, isobutene, 1-butene and the two 2-butenes, and also the saturated hydrocarbons isobutane and n-butane. Customary workup processes practiced worldwide for such C4 cuts include the following steps. First, the majority of the butadiene is removed. When butadiene can be brought to market viably or there is in-house consumption, it is removed, for example, by extraction or extractive distillation. Otherwise, the butadiene is hydrogenated selectively to linear butenes to as full an extent as possible, but at least up to concentrations of from 1 to 0.1% by mass. In both cases, a hydrocarbon mixture (frequently referred to as raffinate I or hydrogenated crack-C4) remains, which, in addition to the saturated hydrocarbons (n-butane and isobutane), includes the olefins (isobutene, 1-butene and 2-butenes). The isobutene present therein is reacted with ethanol to give ETBE. The distillative workup gives rise to ETBE and an ethanolic C4 hydrocarbon mixture from which ethanol is removed by extraction with water.
In industry, isobutene or isobutenic hydrocarbon streams are usually reacted over solid acidic catalysts, for example, over acidic ion exchange resins.
In this reaction, water reduces the activity of the catalyst very greatly and thus lowers the space-time yield for ETBE. In addition, isobutene in the presence of water forms tert-butanol (TBA) as a by-product. In industry, low-water ethanols are therefore used in the preparation of ETBE, especially those that have a water content of below 1% by mass.
The commercial ethanol qualities have a water content of from 5 to less than 1% by mass. The homoazeotrope of ethanol and water still has a water content of 4.4% by mass. Drier ethanols, especially those having a water content of below 1% by mass, can be prepared only at a high level of cost and inconveniently.
An industrial process for dewatering ethanol is azeotropic distillation with an azeotroping agent that forms a ternary minimum heteroazeotrope with ethanol and water. The azeotroping agents used are, for example, heptane, cyclohexane or benzene. A general flow diagram of such a plant and literature references can be found in Ullmann's Encyclopedia of Technical Chemistry, Vol. 9, pages 634-635, 5th Edition. The main disadvantage of these dewatering processes lies in their high operating costs.
A further process for drying ethanol is water removal by pervaporation at membranes (for example, U. Sander, H. Janssen, Industrial application of vapour permeation, Journal of Membrane Science, 61 (1991), p. 113 to 129, Elsevier Science Publishers B. V, Amsterdam; A. H. Ballweg, H. E. A. Brüschke, W. H. Schneider, G. F. Tusel, Pervaporation Membranes, Proceedings of Fifth International Alcohol Fuel Technology 1982, p. 97 to 106; H. E. A. Brüschke, State of Art of Pervaporation, Proceeding of Third International Conference on Pervaporation 1988, p. 2 to 11). In this process, the water to be removed is obtained together with ethanol in vapor form as the permeate. In order to achieve maximum water removal, the pervaporation is carried out in a plurality of membrane modules connected in series, the retentate of one module in each case being the use mixture of the next module. This process is suitable particularly for the dewatering of ethanols with a very low water content (from 1 to 10% by mass). The permeates obtained can be worked up in a column to give the ethanol/water azeotrope which can be recycled into a membrane unit. The main disadvantage of this process lies in the high capital investment for the membrane modules.
In most ETBE plants, there are typically at least two different ethanol streams, specifically the use ethanol (ethanol from another plant or bought-in material) and at least one ethanol stream recovered from an ethanolic C4 mixture by extraction and distillation. The fresh ethanol generally has a water content of from greater than 0 to less than 1 to approx. 5% by mass; the recovered ethanol usually has a water content of from 5 to 12% by mass. Typically, these streams are combined before dewatering.